An air flow rate measuring device disclosed in JP-A-9-287985 measures an intake amount of air flowing in an internal combustion engine. As shown in FIGS. 11A, B, the air flow rate measuring device includes a bypass passage 100, through which intake air partially flows. The air flow rate measuring device includes a sensing unit in the bypass passage 100.
The bypass passage 100 has an inlet 110, which opens in a direction opposite to a flow direction of intake air. Besides, as shown by an arrow in FIG. 11B, the bypass passage 100 is in a shape, in which air flowing from the inlet 110 turns at a substantially right angle. A bent portion is provided midway the bypass passage 100. The bent portion changes air in flow direction. A passage on the upstream side with respect to the bent portion is partitioned by a partition 120 from a passage on the downstream side with respect to the bent portion.
The sensing unit includes a heater element 130 to measure a flow rate of air. The heater element 130 is arranged in the passage on the upstream side relative to the bent portion of the bypass passage 100. However, as shown in FIG. 11A, the heater element 130 is arranged in a direction such that the lengthwise direction of the heater element 130 becomes perpendicular to both side faces forming the bypass passage 100. Here, both the side faces are perpendicular to the flow direction of air. The heater element 130 is arranged lengthwise in the horizontal direction in FIG. 11A. As referred to FIG. 11B, the heater element 130 is arranged in a position, which is biased, i.e., displaced to the side of the partition 120 with respect to the center in the bypass passage 100.
In the above air flow rate measuring device, air flowing from the inlet 110 bends at a substantially right angle in flow direction. Therefore, as shown in FIG. 12B, as flow velocity of air flowing into the bypass passage 100 becomes high, the maximum flow velocity is biased to the side of the partition wall 120 in a flow velocity distribution X shown by the solid line in the passage on the upstream side of the bent portion. That is, flow velocity of air is stabilized while being biased to the side of the partition 120 relative to the center of the passage on the upstream side with respect to the bent portion. Therefore, the heater element 130 is arranged in a position, which is biased to the side of the partition 120 with respect to the center pf the passage, so that measurement accuracy is enhanced.
The above conventional structure is effective when an intake amount of air is large, i.e., a flow velocity of air is high. However, as shown in FIG. 12A, the flow velocity distribution X is formed around the center of the bypass passage 100 on the upstream side of the bent portion in a low flow rate condition, in which an intake amount of air is small.
Accordingly, when a flow rate is low, the flow velocity distribution X becomes in a parabolic shape in the bypass passage 100. Accordingly, when the heater element 130 is arranged in a position eccentrically biased to the side of the partition 120, a flow rate is measured in a region, in which flow velocity is low in the flow velocity distribution X. As a result, measurement accuracy is degraded when a flow rate is low, and a dynamic range of flow measurement becomes small.